1. Field of the Invention
The present invention concerns the transmission of digital data and in particular the transmission of such data by Time Division Multiplex (hereinafter referred to as TDM).
2. Description of Related Art
In TDM Data Transmission Systems a plurality of separate digital signals at various frequencies are combined at a transmission terminal into a single data stream of higher frequency. The original separate signals, known as tributaries, are interleaved at the transmission terminal so that in the final multiplexed signal one or more symbols from one tributary are separated by same from each of the other tributaries until the next symbol(s) of that tributary appear(s). In effect the Multiplexed signal is divided into frames of equal length, each frame being defined by a fixed set of bits known as a frame word and containing, along with various over-head bits, a single symbol/group of symbols from each of the tributaries. At the reception end the interleaved signals have to be "disentangled" and sent on to the appropriate output tributaries.
It will be apparent that any errors occurring during transmission of the Multiplexed data stream can cause extreme problems at the reception end as the breakdown of the Multiplexed data stream into its component parts requires absolute accuracy.
Thus in order to monitor the performance of a digital transmission system it is necessary to detect at the receive terminal any errors produced by the system when in service. The errors can be caused by a number of different factors and at any location along the transmission route. Thus errors can be caused by a faulty dependent repeater or regenerator.
One method by which errors of this nature can be detected utilizes the fact that in some digital transmission systems the line signals are arranged or can be converted into a format resulting in what may be termed constant accumulated disparity signals. An example as to how such signals can be used to detect errors is given in British Patent Specification No. 1536337.
However the ability to use an error detection system such as is proposed in British Patent No. 1536337 is dependent on the nature of the framework or basic structure of the Multiplexed coded signal. These structures are, because of the need to maintain compatible standards internationally, usually defined by an internationally supported body known as the CCITT.
For 2 Megabits traffic, a structure has been defined by the CCITT which is suitable for the purposes of switching, signalling and with minimal extra processing, transmission, with a growing range of support functions being defined to use the spare channel capacity. A further set of recommendations by the CCITT define a frame structure which is even better for these purposes and for bit rates from 2 Mbit/s to many Gbits/s. These recommendations originated in the North American SONET standard, and were agreed at Seoul in February 1988. Hereinafter the new standard will be referred to as SDH.
SDH is based on a module with a bearer rate at 155.52 Mbit/s, carrying a payload of 150.336 Mbit/s. The payload can be formed in a variety of ways, and three particular options are optimised for supporting N. American and European bit rates (1.5 to 45, 2 to 34 Mbit/s, and 140 Mbit/s respectively).
The new structure is designed for low cost switching at many levels of bandwidth, from 64 kbit/s to Gbit/s rates. SDH networks can run more efficiently because bandwidth can be readily allocated down to customer level by remote control, and can be steered around the network in large or small blocks to allow for maintenance, protection and traffic loading.
The introduction of the CCITT recommendations is expected to lead to substantial cost reductions. Direct reductions occur because of the integration of so many functions which becomes possible in one equipment, because of the simplification of equipment interfaces, and because of the economies from having one world standard for manufacturers. Indirect reductions occur because the new switching potential allows optimum allocation of network capacity, with the additional costs of switches being much less than the perceived saving in operating costs, produced for example by not needing site visits to change customer facilities.
However the SDH structure lacks any simple means of locating errors in regenerators. The only available technique is to detect each frame and to compute parity. The higher the rate of data transmission the more and more expensive in terms of heat and power does this solution become. The problem is that frame overheads in SDH do not give enough capacity for effective mark parity. Thus in SDH 38 bytes are allocated for national use. This is equivalent to 1 in 64 bytes and is inadequate with low-cost implementations for the purpose of mark parity.